Night Stalker
It began with blood and death amid the streets of late-night Toronto. Vicki Nelson, formerly of Toronto's homicide detail, now a private investigator, witness the first attack by the force of dark magic that would soon wreak its reign of terror on the unsuspecting city. And as death followed unspeakable death, Vicki becomes more and more deeply enmeshed in an investigation which would see her renewing her stormy relationship with her former police partner, Mike Celluci, even as she teamed up with writer Henry Fitzroy in a desperate attempt to track down the source of the seemingly unstoppable attacks. For Fitzroy, the illegitimate son of Henry VIII, had knowledge of realms beyond the mortal acquired over the centuries during which he'd mastered his own insatiable need--the life-from-death cravings of a vampire.

Henry Fitzroy, had long since learned to survive without killing, learned the skills needed to blend in with the human race. But unless he, Vicki, and Mike could fine the key to conquering the magic-raised menace stalking the streets of Toronto, Fitzroy's true identity might soon be exposed and his life might prove forfeit to the uncontrollable fears of humankind. And without Henry Fitzroy, mere mortals like Vicki and Mike would not long survive against the ancient force of chaos that had been loosed on their world...

What a great book. Lots of action and some scary stuff too. Vicki is a smart woman and the 2 men helping her investigate are just as smart. By the way, one's a cop and one's a vampire.
This series was written in the early '90s way before Charlaine Harris and Kim Harrison starting writing books similar to this series.